1. Field of the Invention
The present invention relates to supports for bearings that guide rotor shafts in turbomachines such as aircraft bypass turbojet engines.
2. Description of the Related Art
Turbojet engine fan blades may be subjected to damage, for example following ingestion of foreign objects such as birds or blocks of ice.
In general, the fan is robust enough to withstand the effects of such an ingestion and to continue to operate, possibly at a lower speed.
However, ingestion of a foreign body can sometimes cause one or more fan blades to break, this being liable to have the effect of causing significant imbalance requiring the engine to stop, and capable of subjecting the structure of the turbojet engine to considerable cyclic forces, at least while the engine is slowing down to its windmilling or autorotation speed, that is to say to its speed of free rotation as a result of its movement through the atmosphere.
In order to avoid imbalance forces being transferred to the structure of the turbojet engine, it has been proposed for the low pressure compressor shaft that bears the fan to be decoupled from the stator.
This shaft is generally radially guided by two bearings supported by the stator. A first bearing, sometimes known as the upstream bearing or thrust bearing, comprises an antifriction ball bearing positioned at the upstream end of the shaft and is supported by an annular support connected to an intermediate casing of the turbojet engine, while a second bearing, commonly known as the downstream bearing, comprises an antifriction roller bearing positioned downstream of the first bearing.
Devices for decoupling the upstream bearing and comprising “rupture” screws for connecting the support of this bearing to the intermediate casing have been proposed, as have other devices comprising a bearing support that has a local striction engineered to allow the support to deform when the applied load exceeds a predetermined load, and other devices proposed have included a corrugated bearing support intended to buckle under a certain load. Devices comprising “rupture” pins in the region of the upstream bearing have also been proposed.
However, when the upstream bearing has been decoupled from the stator it no longer centres the low-pressure compressor shaft and even though the loads resulting from the imbalance are not of such a great magnitude once the autorotation speed has been reached, these loads are then essentially borne by the downstream bearing, with a risk therefore of breaking the latter.
To avoid this disadvantage, devices have been proposed in which the downstream bearing is, for example, mounted on a support that is articulated to allow the shaft to become off-centred without destroying this downstream bearing, but these devices lead to additional mass penalizing turbojet engine performance.
Furthermore, with the known devices and should the low pressure compressor shaft break, the upstream part thereof is no longer axially retained by the upstream bearing, thus potentially endangering the turbojet engine.